Production and measurement of workpieces

ABSTRACT

In a workpiece production method a plurality of nominally similar workpieces are produced in a production process on one production machine. The order or time of production of some of the workpieces on the production machine is recorded. Some of the workpieces recorded are measured at two or more inspection stations. Dimensions or points of one workpiece are measured at one of the inspection stations, and corresponding dimensions or points of another of the workpieces are measured at another of the inspection stations. The results of the measurements of corresponding dimensions or points made at the two or more inspection stations are analysed together, taking account of the order or time of production of the workpieces. An output signal is produced based on the analysing of the results together. The output signal indicates performance of the production machine or of one or more of the inspection stations.

FIELD OF THE INVENTION

This invention relates to the production and measurement of workpiecesor parts, and also to methods and manufacturing systems for suchproduction and measurement. The terms “workpiece” and “part” are usedinterchangeably in this specification.

DESCRIPTION OF PRIOR ART

An automated factory manufacturing system may comprise one or moreproduction machines (such as machine tools) for producing workpieces.Typically these may be produced as a series of nominally identicalworkpieces. The manufacturing system may also comprise one or moreinspection stations for inspecting the workpieces produced. Aninspection station may comprise conventional gauging such as fixturegauges, or even manual gauges such as height gauges or calipers. Or itmay comprise a coordinate measuring machine (CMM) for measuring theworkpieces, or a comparative gauging machine for comparing them with amaster reference. These production and inspection machines may each havea numerical control or computer control, linked by a network to one ormore server computers. An example is seen in U.S. Pat. No. 5,189,624(Barlow et al).

A proportion of workpieces produced on a production machine (or even allworkpieces produced) may be inspected at an inspection station. A servermay schedule workpieces which are to be transferred to an inspectionstation, and may control transfer robots or conveyors for this purpose.

In some prior art examples, the inspection results may simply be a passor fail decision. In the case of fail decision (a rejection), this canbe fed back to allow adjustment of the production machine, so as tocontrol and improve the subsequent production process. Such control ofthe production process is performed manually in the example of U.S. Pat.No. 5,189,624. Alternatively, even in the case of a pass decision, if adimension of a single workpiece has exceeded a control limit, anautomatic feedback may be provided to adjust the production machine,e.g. to update a cutting tool offset by an appropriate percentage of theerror in the dimension. In this case the control limit may be set at alower level than the tolerance limit at which workpieces would berejected. Alternatively, the control limit could be set at the level atwhich workpieces are rejected.

It is known to perform more sophisticated analysis of the inspectionresults of multiple workpieces in the series of nominally identicalworkpieces. For example, a series of measurements of a particulardimension on successive workpieces may be filtered to remove outliers.Alternatively, the series of measurements may be analysed to detect atrend. For example, if the production machine is a machine tool with acutting tool which wears in use, or which is subject to thermal drift,there may be a gradual trend in which the dimension of a feature of theproduced workpieces increases or decreases over time. Such analyses maybe performed after inspecting workpieces in a quality control room orlaboratory, separate from the production machines. Manual correction ofthe production process may subsequently be applied by a skilled machineoperator, but will not have a beneficial effect on workpieces which havebeen produced in the meantime.

U.S. Pat. No. 6,400,998 shows a system in which workpieces produced on amachine tool are inspected on a measuring machine. The measurementresults may be analysed in various ways and fed back to the machinetool.

Analysing trends requires that the workpieces should be inspected at theinspection station in the same order as they are produced by theproduction machine. Otherwise the trend would not easily be observed.Conventionally, this is achieved by sending all the workpieces which areto be inspected from a given production machine to the same inspectionstation, as they are produced.

However, that is a constraint on the production scheduling performed bythe server, which may result in inefficient usage of the productionmachines and inspection stations in the factory. It also requires thatthe speed of the inspection should be sufficiently fast, so that theproduction is not slowed down by the inspection.

US 2010/0228510 discloses a quality information control analysis systemfor analysing defects in products being assembled by a plurality ofassembly machines or installations. When a product is inspected by aninspection machine after assembly, the inspection results may specifythe existence of a defective component and the type of defect that hasbeen detected. The products are not inspected on a measuring machine andaccordingly the inspection results do not include measurement results,so that for example it is not possible to determine a gradual trend inwhich the dimension of a feature of the product increases or decreasesover time. Furthermore, US 2010/0228510 relates to assembly ofalready-manufactured components rather than to a manufacturing systemwhich comprises one or more production machines (such as machine tools)for producing a series of nominally identical workpieces. It is also nottaught that the output from a particular assembly machine can be sent toany available inspection machine.

SUMMARY OF THE INVENTION

The present invention provides a workpiece production method comprising:

producing a plurality of nominally similar workpieces in a productionprocess on at least one production machine;

recording the order or time of production of at least some of theworkpieces on the production machine;

measuring at least some of the workpieces so recorded at two or moreinspection stations; wherein dimensions or points of one workpiece aremeasured at one of the inspection stations, and corresponding dimensionsor points of another of the workpieces are measured at another of theinspection stations; and

analysing together the results of the measurements of correspondingdimensions or points made at the two or more inspection stations, takingaccount of the order or time of production of the workpieces.

In the preferred embodiments of the invention, recording the order ofproduction of the workpieces and taking it into account while analysingthe results enables the use of more than one inspection station forworkpieces produced by a given production machine, and reduces theconstraints noted above on production scheduling. This may enable theproduction scheduling to make more efficient use of the productionmachines and inspection stations in the manufacturing system or factory.

An output signal may then be produced, based on analysing the resultstogether, the output signal indicating performance of the productionmachine or of one or more of the inspection stations. It is possible toassess the production process, or to determine the process capability ofthe production machine or the capability of an inspection machine at aninspection station. For example, the analysis may quantify thecapability of the production machine to manufacture workpieces to apredetermined tolerance.

The output signal may be an alarm signal or a message to a humanoperator if the results are approaching or exceed a predetermined limitvalue or tolerance value. In some preferred embodiments, however, theoutput signal is fed back to the production machine to adjust theproduction process of the production machine. For example, it may be atool offset value to correct production of future workpieces.

The order of production of the workpieces may be derived from the timesof their production. When analysing the results of the measurements,account may be taken of time intervals between the times of productionof the workpieces.

Analysing the results of the measurements may produce an ordered historyof the performance of the production machine over time. The method mayinclude detecting whether there is a trend in the results of themeasurements over time. For example, it may be detected that aparticular dimension is gradually increasing (or decreasing) in sizeand/or detected that the coordinates of a particular point are changingas successive workpieces are produced; this may be an indication that atool is wearing or that the setup of the machine is drifting in someother way. The method may include detecting whether there is a trend inthe performance of the production machine over time. It may detect whena particular dimension of a workpiece or the coordinates of a particularpoint has exceeded a control limit. Or it may filter a series ofmeasurements of a particular dimension or point on successive workpiecesto smooth the results or to remove outliers, and optionally detect whenthe filtered series of measurements has exceeded a control limit. Theresults of the measurements may be analysed statistically to determinethe process capability of the production machine which produced theworkpieces. Any of the above assessments of the production machine maybe directed to individual tools or tool turrets of the machine, or tothe machine as a whole.

There may be a plurality of production machines, and each productionmachine may be associated with two or more inspection stations. Eachinspection station may measure dimensions or points on workpiecesproduced only by one particular production machine, for example in aproduction cell where the two or more inspection machines are arrangedwith that production machine. Or one or more of the inspection stationsmay measure dimensions or points on workpieces produced by more than oneof the production machines, to form a more flexible manufacturingsystem.

In another embodiment of the invention, the analysis of the resultscomprises comparing the results of the measurements of correspondingdimensions or points made at the two or more inspection stations, andthe output signal is provided if the results from one of the inspectionstations differ by more than a predetermined amount from the results ofone or more of the other inspection stations. This may allow anassessment of the capability of an inspection machine at the inspectionstation.

The invention also includes manufacturing systems arranged to operateany of the above methods. Such a manufacturing system may comprise: oneor more production machines; two or more inspection stations; and acontrol system for scheduling the production of workpieces on the one ormore production machines and the inspection of workpieces so produced atthe inspection stations; wherein the control system is configured tooperate a method as described above.

The or each production machine may be a machine tool. The or eachmachine tool may perform a machining operation in order to produce aworkpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a manufacturing system arrangement in afactory, including production machines and inspection machines;

FIG. 2 is a schematic diagram showing a first embodiment of a scheme forinformation flow in the manufacturing arrangement of FIG. 1;

FIGS. 3 and 4 correspond to FIG. 2, showing second and third embodimentsof such information flow;

FIG. 5 shows a part record used in these embodiments;

FIG. 6 is a flowchart showing the operation of a process control modulewhich runs in a data server in FIGS. 1-4;

FIG. 7 is a graph illustrating measurement results received by theprocess control module; and

FIGS. 8 and 9 are graphs illustrating possible analyses performed by theprocess control module.

DESCRIPTION OF PREFERRED EMBODIMENTS

The manufacturing arrangement of FIG. 1 includes a number of productionstations, each comprising a computer numerically controlled (CNC)production machine 10,12,14,16 for producing parts (workpieces). Theproduction machines may use any manufacturing technology. They may bemachine tools such as milling machines, lathes, mill-turning centres,machines for grinding, drilling, laser cutting, lapping, honing,polishing, etc. Or they may be coating machines, forges, presses, oradditive manufacturing machines (3D printing). The exact number of thesemachines is unimportant; there may be one or more. Any combination ofdifferent types of production machine may be present, or they may all beidentical.

Each production machine is controlled by a respective controller 11, 13,15, 17 which may comprise a conventional CNC control. Optionally, any orall of the controllers may include a separate computer in communicationwith the CNC control.

The manufacturing arrangement also includes two or more inspectionstations, each comprising an inspection machine, preferably a CNCgauging machine 20,22,24 for inspecting parts (workpieces) produced bythe production machines. A suitable flexible comparative gauging machineis sold by the present applicants Renishaw plc under the trade markEQUATOR. As described in our earlier international patent applicationno. WO 2013/021157, which is incorporated herein by reference, thisgauging machine has a motorised structure with a non-Cartesian geometry.This moves a probe in three dimensions relative to a productionworkpiece, in order to compare the production workpiece to a masterreference workpiece. Each gauging machine is controlled by a respectivecomputer controller 21, 23, 25.

Instead of these gauging machines, the inspection stations couldcomprise other dimensional measuring equipment, such ascomputer-controlled coordinate measuring machines (CMMs) or inspectionrobots. Alternatively, they may comprise gauging fixtures or jigs, inwhich gauges with LVDT or other transducers are custom-designed tomeasure specific dimensions of the workpieces. The measurement resultsof these gauges may be fed automatically or manually into the respectivecontroller 21, 23, 25. It is also possible to have inspection stationsin which workpieces are measured manually using conventional hand-heldgauges such as height gauges or calipers, the results being fed into thecontroller 21, 23, 25 or into a terminal of a common server computer.

The manufacturing arrangement further includes a transport system(indicated schematically by an arrow 26), for transferring parts(workpieces) from any of the machine tools 10,12,14,16 to any of thegauging machines 20,22,24. Here they can be inspected for conformance tospecified dimensional tolerances. The transport system could comprisecomputer-controlled robots, vehicles or conveyors, or could simplyinvolve the manual transfer of workpieces or pallets of workpieces. Itmay be part of a larger transport system which also supplies raw billetsor castings for machining to the machine tools, and/or removesworkpieces after manufacture or after inspection. If necessary, it mayreturn a workpiece to a machine tool for re-work after inspection.

A job scheduling server 28 is also provided. A program or softwaremodule in this server 28 is responsible for scheduling the production ofworkpieces, and is connected to the CNC controllers of the machine toolsand gauging machines by one or more data buses 30. The scheduling server28 also controls the transport system 26, e.g. for transferringworkpieces between the machine tools and the gauging machines whenrequired. For example, the scheduling server 28 may take the form of aprogrammable logic controller, as used conventionally to control aproduction cell with multiple machine tools, but with differentprogramming as described below.

The scheduling server 28 may provide the machine tools and gaugingmachines with the necessary CNC part programs for machining andinspecting each particular design of part (workpiece) to be produced, asthey are required.

Alternatively, these part programs may be stored in the controllers ofthe machine tools and gauging machines, and selected for use on thebasis of instructions received from the scheduling server 28.

Typically, the scheduling server 28 may schedule that a series ofnominally identical parts (workpieces) of a particular design are to beproduced on (say) the machine tool 10. Conventionally, it could theninstruct that each of these workpieces is to be transferred to thegauging machine 20 and schedule its inspection on that machine. Or itcould schedule that a regular sample of the workpieces is inspected(e.g. that every 10th workpiece is inspected; or that a workpiece isinspected after a given time period has elapsed, such as one workpieceper hour).

Inspection of each workpiece on the gauging machine 20 produces multipledimensional measurements which are passed back to a data server 29 onthe bus 30 and stored. If a workpiece is determined to be out oftolerance, either by the data server 29 or by the controller 21, 23, 25of the gauging machine, then a message may be passed to the schedulingserver 28. This message may specify that the out-of-tolerance workpieceshould be rejected or scheduled for re-work.

In addition to rejection or re-work, the data server 29 also has asoftware module programmed to perform process control. It examinestrends in the inspection measurement results from successive workpiecesas they are inspected. It may for example determine a trend that aparticular dimension is gradually increasing in size as successiveworkpieces are produced. That may be caused by wear of an associatedcutting tool in the machine tool 10, or by gradual thermal growth of themachine tool or of the raw material stock or billets or castings fromwhich the workpieces are machined. The data server 29 can then feed backan updated offset value for the corresponding cutting tool over the bus30 to the CNC control of the machine tool 20. This corrects themachining process to ensure that future workpieces in the series remainin tolerance.

Of course, the scheduling and the storage and processing of inspectionresults may all be performed on a single server running correspondingsoftware modules, rather than on separate scheduling and data servers28, 29 as described.

Alternatively, in a large factory, there may be multiple productioncells, each with its own scheduling server 28 controlling a plurality ofmachine tools and gauging machines. The measurement results from thegauging machines in multiple production cells may be sent for processingto a common data server 29 serving all the production cells.

As another alternative, in a large factory there may be two or more dataservers 29, with one or more scheduling servers 28. These shouldpreferably have appropriate network links and software to exchange data.The data may be stored in a shared database (or in an underlying datalayer in the software architecture) so that measurements of workpiecesmay be processed by any of the data servers. Alternatively allmeasurements of workpieces from a given production machine may bereceived and processed by a particular one of the data servers,whichever inspection machine they were measured on.

If desired, the scheduling server 28 and/or data server 29 may belocated remotely from the factory, scheduling production and/orreceiving the measurement results over a wide area data network or theinternet. Such remote servers may even schedule production and/orreceive and process measurement results from production cells in two ormore factories in different locations. If the data server 29 is locatedin the factory, it is possible for the data which it stores to bemirrored on a remote server, to aid and assess quality control of theproduction process.

A server 28 which schedules the production and controls the productionmachines and gauging machines is not essential. The productionscheduling, the usage of the various machines and the transfer ofworkpieces between machines may be all be decided and performed by humanoperators instead.

In order to determine trends in the measurements, the workpieces from agiven machine tool 10 could all be presented to the same gauging machine20, in the same order that they are produced by the machine tool 10.However, that would be a limitation on the flexibility of the overallproduction in the factory. It may occur that the gauging machine 20 isbusy when a workpiece produced by the machine tool 10 is required to beinspected. For example, the time required to inspect a workpiecetypically differs from the time required for its production.Furthermore, the gauging machine 20 might be required for inspection ofother workpieces produced on one of the other machine tools 12,14,16.These other workpieces may be of the same or a different design to theseries being produced by the machine tool 10.

In the present embodiments of the invention, therefore, when a workpiecefrom the machine tool 10 is to be inspected, the scheduling program inthe server 28 is configured to send it to whichever gauging machine20,22,24 is least busy or most appropriate, and to supply or instructthe use of the appropriate inspection program by that machine. Or ifthere is a manual transport system, a human operator may be sent aninstruction to transfer the workpiece manually to an appropriate gaugingmachine. In a less sophisticated system, the human operator may decidewhich gauging machine to use.

As workpieces are sent to different gauging machines, a record is keptof the order of their production. For example, this may be a timestampof the time of production of each workpiece on the machine tool 10.Examples of this are discussed below in relation to FIGS. 2-4.

FIG. 2 is a schematic diagram showing a first embodiment of a scheme forthe information flow between the scheduling and data servers 28, 29, thecontroller 11 of one of the machine tools 10, and the controllers 21,23, 25 of several gauging machines 20, 22, 24.

The scheduling program or module in the server 28 instructs thecontroller 11 of the machine tool 10 to produce parts (workpieces) bysending an instruction (including a part number) to the machine toolcontroller 11, over the bus 30 (FIG. 1). The part number specifies thedesign of the part which is to be produced, for example corresponding tothe number of a design drawing or computer-aided design (CAD) file ofthe part. The controller 11 of the machine tool 10 then loads and runs acorresponding CNC part program for producing parts to the specifieddesign. It may load the part program from its own internal storage, orit may receive it with other data over the bus 30.

When the machine tool 10 is about to produce a new part (workpiece), itscontroller 11 sends a new part request 34 over the bus 30 to thescheduling server 28. This request includes a timestamp for the part,generated by the controller 11 of the machine tool 10.

When it receives the request 34, the scheduling server 28 sets up a newcentral record for the part, with fields for various items ofinformation, as shown in FIG. 5. These fields include:

A new, unique workpiece/part identity number (part ID) generated by thescheduling server 28. This identifies the individual part (workpiece)which is about to be produced.

The timestamp contained in the request 34.

The part number which refers to the design of the part. This may forexample have been confirmed by the controller 11 in the request 34.

An identity number MT-ID for the machine tool 10. Again, this may forexample have been confirmed by the controller 11 in the request 34.

The scheduling server 28 then transmits a response 36 over the bus 30back to the controller 11 of the machine tool 10. This response 36includes the part ID which has been generated.

The machine tool then machines the part (workpiece) under CNC control,in the conventional manner. When it has finished, the scheduling programmay instruct the transport system 26 to transfer the part to one of thegauging machines 20, 22 or 24 for inspection. Or if there is a manualtransport system, it may send an instruction to a human operator. Asmentioned above, the schedule may require all parts to be inspected, ora regular sample of them, and suitably the scheduling server isprogrammed with the flexibility to choose whichever of the gaugingmachines is least busy or most appropriate.

As indicated at 38, the part ID of the part is transferred with the partfrom the machine tool 10 to the gauging machine. This may be done bylabelling the part (or a pallet or fixture on which it is transferred)at a labelling station 50 connected to the machine tool controller 11.For example, the labelling station may produce a machine-readablebarcode label including the part ID. This label may be attached to thepart or to its pallet or fixture either automatically by a robot, ormanually by a human operator.

When the machined part reaches the gauging machine 20, 22 or 24, abarcode reader 52 connected to the gauging machine controller 21, 23 or25 reads its part ID from its label. As indicated at 39, the gaugingmachine controller then sends the part ID over the bus 30 (FIG. 1) tothe scheduling server 28. The server 28 looks up the corresponding partrecord (FIG. 5) and as indicated at 40 it returns the part number whichspecifies the design of the part (and optionally it may return the wholepart record).

The gauging machine controller then loads a corresponding part programfor inspecting parts of the specified design. It may load the partprogram from its own internal storage, or it may receive it from theserver 28 over the bus 30. It runs the program to inspect the part,measuring specified dimensions or the coordinates of specified pointsand producing a set of inspection results. It transmits these over thebus 30 to the process control module in the data server 29. This isindicated at 41 in FIG. 2. The part ID and optionally the part numberare also transmitted over the bus 30 together with those results.

FIG. 6 is a flowchart showing an example of the process control modulewhich runs in the data server 29. In step 54, when it receives a set ofinspection results, it requests the scheduling server 28 to send therecord information for the part, as identified by the accompanying partID. As shown in FIG. 5, this part record information includes theidentity number MT-ID of the machine tool which produced the part, andthe timestamp generated at the time of production. It may also includethe part number, as an alternative to transmitting it with theinspection results from the gauging machine. It will be recalled thatthe part number specifies the design of the part and the CNC partprogram used to machine it.

Alternatively, instead of the step 54, the scheduling server 28 maypreviously have returned the whole part record to the gauging machinecontroller. In this case, the gauging machine controller transmits thewhole part record on to the data server 29 with the inspection results(at 41 in FIG. 2). There is then no need to request the part recordinformation in step 54.

In step 56, the set of inspection results is inserted into a databasekept in the data server 29, together with the corresponding part recordinformation. This database comprises a historical record of theinspection results and part record information for all parts made by allthe machine tools 10, 12, 14, 16 over a required period of time. Theseresults may have been produced by any of the gauging machines 20, 22,24, and an identity number of the relevant gauging machine may also beincluded in the record.

FIG. 7 shows the measurement results of a particular dimension orcoordinate point of a particular feature of the nominally identicalparts produced by one of the machine tools (e.g. the machine tool 10).They are shown in the order they are received by the process controlmodule in FIG. 6. Bars 70, 72, 74 represent the measurements of theparticular dimension or point from successive nominally identical parts,which have been respectively taken on different gauging machines 20, 22,24.

In step 58, the process control module then uses the part number andMT-ID of the results set just received to interrogate its database. Itretrieves a predetermined number of the most recent sets of inspectionresults for parts having that part number and MT-ID. These represent themost recent nominally identical parts of that design produced by themachine tool concerned, e.g. by the machine tool 10 in the presentexample. The number of sets of results thus retrieved for processing ispredetermined by the user according to the manufacturing process and thetolerance requirements of the parts concerned. For example, the mostrecent 10 or 20 sets of results may be selected. Of course, near thestart of a production run fewer sets of results will be available.

Preferably the sets of results in the database are indexed on thetimestamp, so that in step 58 they are retrieved in the order in whichthe machine tool produced the parts. Otherwise, they are sorted on thetimestamp in a sorting step 60, if necessary. FIG. 8 shows the samemeasurements as FIG. 7, arranged in the order of their production on themachine tool 10. Since the order of production is taken from the storedtimestamps generated by the particular machine tool 10, it does notmatter if the parts were not measured in the order of production,perhaps because they were measured on different gauging machines 20, 22,24 at different times. This may occur, for example, if there is a queueof inspection jobs at one of the gauging machines, so the schedulingserver 28 has rescheduled subsequent inspection jobs to a differentmachine.

Having got the sets of results in the order of production of the parts,the process control now proceeds in step 62. Here the sets of resultsare analysed in that order, to produce an ordered history of theperformance of the production machine or of the relevant tool or toolturret of the production machine which machined the feature concerned.This is done according to pre-set rules which again depend on themanufacturing process and the tolerance requirements of the partsconcerned. Suitable rules are known to those skilled in this field. Somepossible rules are illustrated graphically in FIG. 8.

In FIG. 8, the broken line T denotes a maximum tolerance limit for thedimension or coordinate point concerned. The successive measurementsexhibit a trend increasing towards the tolerance limit T. This may be atrend in the overall performance of the machine tool 10 or otherproduction machine, or in the performance of a tool-holding turret ofthe machine tool, e.g. caused by thermal drift. Or there may be a trendcaused by gradual wear affecting the performance of an individualcutting tool used to machine the parts. The broken line L denotes apredetermined lower control limit, chosen to enable correction of theproduction process before the tolerance limit is exceeded, so thatproduction of in-tolerance parts can continue uninterrupted.

One possible pre-set rule may simply assess whether the measureddimension or point coordinates have exceeded the control limit L. InFIG. 8 this is true of the fifth measurement bar. A more sophisticatedrule analyses the successive measurement results statistically, e.g.using a least squares analysis. This may examine the results for a trendas indicated by the line 78. The rule may be triggered when such a trendis detected, according to suitably chosen criteria such as detectingwhen the slope of the line 78 exceeds a predetermined value. Or anotherpossibility is that the rule may assess whether and when the trend willexceed the control limit L. In FIG. 8 this is true of the sixthmeasurement bar, but may be predicted from earlier measurements. Otherpossible rules may detect a decreasing trend in the measurements, orwhether and when a decreasing trend (or an individual measurement)exceeds a predetermined control limit in a negative direction, before aminimum tolerance level is reached. Other possible rules filter theseries of measurements to smooth the results or to remove outliers inthe results which do not contribute to a general trend. This may be donebefore determining whether the filtered series of measurements exceedsthe control limit or exhibits a trend.

Step 64 in FIG. 6 makes a decision, depending on the outcome of theanalysis in step 62. If the rule has not been triggered, the programdoes nothing (step 66) and just loops back to the beginning to awaitfurther inspection results.

If the rule has been triggered, then corrective action is required. Forexample, the step 64 may generate a control signal or value, such as anew tool offset. The new tool offset could for example be a percentageof the error in the measured dimension, arranged in a sense to counterthe detected trend. This is fed back to the machine tool 10 in step 68,as shown at 80 in FIG. 2, and the program again loops back to thebeginning to await further inspection results. In this case, the newtool offset adjusts the cutting tool of the machine tool 10 which isresponsible for cutting the part feature of which the dimension has beenanalysed. In this way, the data server 29 produces a control signal orvalue which is used to adjust the production process of the machinetool, to ensure that it continues to produce good parts within thetolerance limit T.

Other feedback actions are possible. For example, if the analysis showsthat the tolerance limit T has been exceeded suddenly and unexpectedly,indicating that the cutting tool has broken, the machine tool 10 may beinstructed to substitute a replacement cutting tool for futureproduction. The data server 29 then also instructs the scheduling server28 that the out-of-tolerance workpiece is to be rejected or re-worked.It is also possible that the step 64 could just produce an alarm or senda message to request action by a human operator to investigate theproblem with the machine tool.

It is also possible to perform a statistical analysis in the analysisstep 62, giving statistical process control automatically in real timeon the factory floor, rather than as a result of a subsequent analysisin a quality control room or laboratory. Such statistical processcontrol may determine the process capability of the production machineor of a tool or tool turret of the machine, that is its capability toproduce parts to a predetermined desired tolerance e.g. in terms of aknown process capability index such as C_(pk), C_(p) or P_(pk). This maysimply be output as a management report, or it may be used to feed backto adjust the production process as described above. Or it is possiblethat the production machine may be fully capable of producing parts tothe required tolerance, but offset from the nominal required dimensionalvalues. In this case a correction is fed back to adjust the productionmachine to remove the offset.

FIG. 8 shows the measurement bars 70, 72, 74 placed simply in the orderof production on the machine tool 10, so they are spaced equally. Thismay be appropriate, for example, if tool wear is anticipated and is tobe monitored. In the analysis step 62, it is instead possible to monitortrends etc while taking into account the actual recorded time ofproduction and the time intervals between the times of production ofeach part on the machine tool 10. In this case, the spacing between themeasurement bars will be uneven. This may be appropriate, for example,if changes caused by a gradual temperature drift are to be monitored.

FIG. 9 illustrates a further example of uneven spacing of themeasurement bars, with gaps 75 between measurements. One reason for theuneven spacing is that the analysis step 62 has detected and filteredout an outlier 76, which is not taken into account. Other reasons forthe gaps 75 are that the machine tool may have suffered some downtime.And/or some inspection results may be unavailable because one of theinspection stations is busy and they have been held up. Nevertheless,the analysis proceeds continually, detecting trends and performing otheranalysis rules based on the measurement results which are available. Ofcourse, the spacings may also be uneven simply because they take accountof the time intervals between the actual recorded times of production ofthe workpieces.

It will be recognised that FIGS. 6-9 relate to a set of measurementresults for just one of the dimensions or points of the nominallyidentical workpieces. In practice, the dimensions or coordinate pointsof multiple features of the workpieces may be measured, giving multiplesets of such measurement results. Each set of measurement results may beassessed in the same manner, with feedback 80 to the machine toolcontroller as appropriate. However, it may not be necessary to providefeedback from all the sets of measurement results. For example, if thevariability of the machine tool's production process arises from toolwear, this may be assessed and corrective feedback may be provided fromjust one of the sets of measurement results affected by thecorresponding worn tool. If the variability arises from thermal growth,it may be possible to assess this and provide corrective feedback fromonly one or only a few of the sets of measurement results (correspondingto only one or only a few of the measured nominally identical features).

As mentioned above, the scheduling server 28 and/or data server 29 maybe located remotely from the factory. Even in this case, any of theabove feedback actions may be based upon analysis in the remote serverand an output signal fed back from the remote server to adjust theproduction machine in the factory.

FIG. 3 shows a second embodiment of a scheme for the information flow.In most respects it is similar to FIG. 2, so only the modifications willbe described.

As shown in broken lines in FIG. 1, in this embodiment a manufacturinginformation system (MIS) 42 is associated with a production cellcomprising the machine tools 10, 12, 14, 16 and the gauging machines 20,22, 24. The MIS 42 may comprise a programmable logic controller.

This arrangement is suitable if the scheduling server 28 serves multiplesuch production cells in a larger factory, each production cell havingits own manufacturing information server. It is also suitable if thescheduling server 28 serves several factories remotely, in which casethe MIS 42 may serve several production cells.

The MIS 42 acts as the master data store for the part records seen inFIG. 5, rather than the scheduling server 28. The new part request 34from the controller 11 of the machine tool 10 is sent to the MIS 42,with the corresponding timestamp. The MIS 42 then generates the partrecord (FIG. 5) including the part ID, and returns the part ID in theresponse 36. On receiving a part to be inspected and reading itsbarcode, the gauging machine controller 21, 23, 25 requests and receivesthe part number (and possibly the inspection part program) from the MIS42, as indicated at 39, 40, rather than from the scheduling server.However, if desired, the scheduling server may keep a copy of some orall of the part record information. For example, the scheduling servermay send the part number and/or inspection part program to the gaugingmachine controller when it instructs the transport system 26 to deliverthe part to a particular gauging machine, so that the gauging machinecontroller does not need to request it.

Subsequently, when the data server 29 receives the measurement resultsfor a part from one of the gauging machines, in step 54 of FIG. 6 itsends the part ID to the MIS 42 rather than to the scheduling server 28,to request the part record (with the timestamp). This request is shownat 44, and the return of the part record from the MIS, including thetimestamp, is shown at 46. Similarly, the gauging machine controllersends the part ID to the MIS 42 to request the part number and possiblythe inspection program for the part.

FIG. 4 shows a third embodiment of a scheme for the information flow.Again it is similar to FIGS. 2 and 3, so only the modifications will bedescribed. In this example, when the scheduling server 28 instructs thecontroller 11 of the machine tool 10 over the bus 30 to produce a newpart, the controller 11 generates the part ID and the timestampinternally, as indicated at 48. The labelling station 50 then encodesall of the part ID, the timestamp, the identity number MT-ID of themachine tool 10 and possibly the part number into the barcode label.This is affixed to the part or to its pallet or fixture, and transferredtogether with the part to whichever gauging machine 20, 22 or 24 isinstructed by the scheduling server. All this information is read by thebarcode reader 52 and the gauging machine controller 21, 23 or 25 passesit on to the data server 29 with the measurement results. The dataserver then has no need to make a separate request 54 to the schedulingserver 28 for this information. If the information includes the partnumber and the gauging machine controller has a local copy of thecorresponding inspection part program, then neither does the gaugingmachine controller need to request information from the schedulingserver 28.

Similarly, in FIGS. 2 and 3 it is possible for the timestamp and/or theMT-ID and/or the part number to be encoded in the barcode label andtravel with the part, being read by the gauging machine barcode readerand passed to the data server by the gauging machine controller.

In the above description of FIGS. 2 and 3, the measurement results havebeen ordered using a timestamp generated by the controller 11 of themachine tool 10. However, if desired, this timestamp may be generated bythe scheduling server 28 (or by the manufacturing information system 42in FIG. 3) and passed to the machine tool with the part ID on receipt ofthe new part request 34. Wherever it is generated, it is again possiblefor the labelling station 50 to encode it into the barcode which travelswith the part, and for it to be read and passed on to the data server bythe gauging machine controller.

In the above embodiments, the labelling station 50 has produced amachine-readable barcode label bearing the part ID and/or the timestampand possibly other information, which is affixed to the part or to itspallet or fixture and travels with it from the production machine to theinspection station. However, any other suitable machine-readabletechnology can be used. For example, a USB or flash memory or RFIDmemory device may be incorporated in the pallet or fixture carrying thepart, or may be attached to the part itself. The labelling stations 50and the barcode readers 52 connected to the controllers of the machinetools and gauging machines may then be replaced by USB or RFID writingand reading devices respectively. This may be particularly suitable ifmore information is to be written than will fit in a barcode, e.g.including the timestamp, MT-ID and part number.

The above embodiments have used a timestamp to track the order ofproduction of the parts. However, instead of this, the part ID maysimply comprise a unique, sequentially allocated serial number,generated by the scheduling server 28, the MIS 42 or the machine toolcontroller itself. Being allocated sequentially, this similarlyindicates the order of production of the parts on the machine tool 10.In the process control module (FIG. 6) it can be used in the same way asthe timestamp to index or sort the records of the parts produced by themachine 10 into their order of production. The machine tool identitynumber MT-ID (and possibly the part number) may also be incorporatedinto the part ID with this serial number. This is particularly suitableif the part ID is generated by the machine tool controller. Once again,in this case it may not be necessary for the data server to request thepart record at step 54 in FIG. 6, as it already has the informationneeded in the part ID.

So far, the process control has been described in respect of a series ofnominally identical parts (workpieces) produced on one machine tool 10,using inspection results from two or more gauging machines 20,22,24.This has an advantage of flexible scheduling, since any if one of thegauging machines is busy the parts may be inspected on another gaugingmachine. There is also an advantage of redundancy. If one gaugingmachine goes out of action, then the scheduling server 28 can switch theinspection of future parts to other gauging machines.

The inspection results may also include an identity number for thegauging machine on which they were produced. It is then also possiblefor the process control module in the data server 29 to include a rulewhich compares the inspection results from different gauging machines.If it is found, for example, that the results from one of the gaugingmachines differ significantly from the results of other gaugingmachines, when measuring nominally identical parts produced by the samemachine tool, then the process control module may diagnose that aproblem lies with that gauging machine rather than with the machinetool. For this purpose, the analysis of the results comprises comparingthe results of the measurements of corresponding dimensions or pointsmade on the separate inspection machines, and the output signal isprovided if the results from one of the inspection machines differ bymore than a predetermined amount from the results of one or more of theother inspection machines. Again, the data server may then instruct thescheduling server to switch the inspection of future parts to othergauging machines. It may also trigger an alarm or send a message to ahuman operator to investigate the problem, e.g. whether the gaugingmachine concerned has drifted out of specification. Or it may adjust theresults obtained from the gauging machine whose results differ, theresults being adjusted by an amount which depends on the output signal.

Of course, the other machine tools 12,14,16 will also be producingparts, perhaps to the same design as the parts produced on the machinetool 10, or perhaps to a different design (different part number).Records are kept and information including timestamps, part IDs andmachine tool identity numbers are generated and transferred in the sameway as above. The scheduling program in the server 28 is programmed toschedule their inspection in the same way, on whichever of the gaugingmachines 20,22,24 is most convenient or efficient. So there is a furtheradvantage of flexibility, since any part may be inspected on any gaugingmachine. And the process control module of the data server 29 thenperforms process control for each of those machine tools 12,14,16 in thesame way, noting which machine tool produced each part and their orderof production on that machine tool.

There is a further redundancy advantage in this latter case, if one ofthe machine tools goes out of action (perhaps with a problem detected bya rule in the process control module). There is no gauging machinededicated to that machine tool which would also become unproductive. Thescheduling server can switch the production of parts to other machinetools, and it can continue to allocate the inspection work amongst allthe gauging machines.

1. A workpiece production method comprising: producing a plurality ofnominally similar workpieces in a production process on at least oneproduction machine; recording the order or time of production of atleast some of the workpieces on the production machine; measuring atleast some of the workpieces so recorded at two or more inspectionstations; wherein dimensions or points of one workpiece are measured atone of the inspection stations, and corresponding dimensions or pointsof another of the workpieces are measured at another of the inspectionstations; analysing together the results of the measurements ofcorresponding dimensions or points made at the two or more inspectionstations, taking account of the order or time of production of theworkpieces, and producing an output signal based on the analysing of theresults together, the output signal indicating performance of theproduction machine or of one or more of the inspection stations.
 2. Amethod according to claim 1, wherein the order of production of theworkpieces is derived from the times of their production.
 3. A methodaccording to claim 2, wherein analysing the results takes account oftime intervals between the times of production of the workpieces.
 4. Amethod according to claim 1, wherein analysing the results of themeasurements produces an ordered history of the performance of theproduction machine or of a tool or tool turret of the production machineover time.
 5. A method according to claim 1, wherein analysing theresults of the measurements includes detecting whether there is a trendin the results of the measurements over time, for example that aparticular dimension is gradually increasing in size as successiveworkpieces are produced.
 6. A method according to claim 1, whereinanalysing the results of the measurements includes detecting whetherthere is a trend in the performance of the production machine or of atool or tool turret of the production machine over time.
 7. A methodaccording to claim 1, wherein analysing the results of the measurementsincludes detecting when a particular dimension of a workpiece or thecoordinates of a particular point has exceeded a control limit.
 8. Amethod according to claim 1, wherein analysing the results of themeasurements includes filtering a series of measurements of a particulardimension or point on successive workpieces to smooth the results or toremove outliers.
 9. A method according to claim 8, including detectingwhen the filtered series of measurements has exceeded a control limit.10. A method according to claim 1, wherein the results of themeasurements are analysed statistically to determine the processcapability of the production machine or of a tool or tool turret of theproduction machine which produced the workpieces.
 11. A method accordingto claim 1, wherein each workpiece is assigned an identity number inmachine readable form, which travels on or with the workpiece from theproduction machine to the relevant inspection station, and the order ortime of production of the workpiece is recorded centrally with theworkpiece identity number.
 12. A method according to claim 1, whereinthe time of production of each workpiece, or a number indicating theorder of production of the workpiece, is recorded in machine readableform and travels on or with the workpiece from the production machine tothe relevant inspection station.
 13. A method according to claim 1,wherein the production machine and the inspection stations are locatedin a factory, and the analysis of the results of the measurements andthe production of the output signal take place at a location remote fromthe factory.
 14. A method according to claim 1, including feeding theoutput signal back to the production machine to adjust the productionprocess of the production machine.
 15. A method according to claim 14,in which the output signal which is fed back to the production machinecomprises a tool offset value to correct production of futureworkpieces.
 16. A method according to claim 1, wherein the output signalprovides a measure of process capability of the production machine or ofa tool or tool turret of the production machine.
 17. A method accordingto claim 1, wherein the output signal includes an alarm signal or amessage to a human operator produced if the results are approaching orexceed a predetermined limit value or tolerance value.
 18. A methodaccording to claim 1, wherein analysing the results comprises comparingthe results of the measurements of corresponding dimensions or pointsmade at the two or more inspection stations, and wherein the outputsignal is provided if the results from one of the inspection stationsdiffer by more than a predetermined amount from the results of one ormore of the other inspection stations.
 19. A method according to claim18, including adjusting results obtained from the said one of theinspection stations whose results differ, the results being adjusted byan amount which depends on the output signal.
 20. A method according toclaim 1, wherein there are a plurality of production machines, eachproduction machine being associated with two or more inspectionstations, and wherein each inspection station measures dimensions orpoints on workpieces produced only by one particular production machine.21. A method according to claim 1, wherein there are a plurality ofproduction machines, each production machine being associated with twoor more inspection stations, and wherein one or more of the inspectionstations measures dimensions or points on workpieces produced by morethan one of the production machines.
 22. A manufacturing systemcomprising: one or more production machines; two or more inspectionstations; and a control system for scheduling the production ofworkpieces on the one or more production machines and the inspection ofworkpieces so produced at the inspection stations; wherein the controlsystem is configured to operate a method according to claim
 1. 23. Amethod or system as claimed in claim 1, wherein the or each productionmachine is a machine tool.